I am not able to find the @ServiceReference annotation (org.springframework.osgi.extensions.annotation) using Virgo. I thought that Virgo 2.1 was built against DM 2.1.0M1. I do not find this package available for import.

Does DM annotations usage require additional bundles to be installed? When I search the Spring OSGi Repository against this package I find references only to Spring DM Annotations Extensions 1.1.0 to 1.1.3, but this does not feel right if my assumption of DM 2.1.0M1 is correct.

Currently Virgo is built against Spring DM 1.2.1. This should be the reason why these annotations are missing.
Adoption of newer Spring DM version is not planned, but instead we intend to move directly to Gemini Blueprint which is based on the latest release of Spring DM. The issue here is that Gemini is still in incubation and its release is scheduled somewhere around Q1 next year, so this is when Virgo should consume it.